The present invention relates to a photoelectric smoke sensor for detecting a fire from a reduction in the amount of received light caused by smoke flowing into a space between a light emitting device and a light receiving device.
In a photoelectric smoke sensor having a construction in which a light emitting device and a light receiving device are separately disposed, an optical system can be accumulatively coated with dust, and can be stained and soiled through its long use. Also, the light emitting and receiving elements associated with the optical system necessarily age. These factors mainly reduce the amount of light in the optical system to often cause a false alarm.
Attempts have been made to compensate for the attenuation of the received light caused by factors other than smoke. In Japanese Laid-open patent application Nos. 16,481/74 and 37,084/77, and in Japanese patent application Publication No. 37,789/77, fire detection is made by comparing light receiving signals stored in an analog or digital manner in a memory with receiving light signals received periodically. At the same time, a change in the amount of the received light, caused by dust and dirt or soil attached to the optical system, is detected at given intervals of time, thereby compensating for the stored amount of receiving signals.
The proposal is effective and useful as long as a lower correction is required. As mentioned above, the proposal corrects the memory contents as a reference of comparison so as to follow an amount of attenuation of the light due to the dust and soil attached to the optical system and compares the corrected signal with a light amount measured at a present time. Therefore, as the dust or soil of the optical system increases, the received light per se at the present time becomes small, so that the S/N ratio in the case of signal processing deteriorates. In this system, the memory content is corrected to the receiving light signal at the present time at a predetermined period of time. Accordingly, the memory content itself contains a receiving light signal under a worse S/N ratio condition. As a result, the receiving light signal at the present time having a worse S/N ratio is compared with the memory content having a worse ratio, so that compensation accuracy increases as the dust or soil of the optical system increases. Therefore, a possible range of the compensation of the memory contents has a certain limit. In this proposal, the memory contents immediately after the smoke sensor is installed is corrected with the progression of the aging of the optical system. When an inspection of the smoke sensor is required, it is impossible to know what amount of correction has been made or if the correction is within a proper correcting range or not. In the worst case, accordingly, there is a problem that a false alarm or non-fire alarm is issued.
In addition, this system automatically performs the correction and storage of the memory content, which is automatically corrected at a predetermined period. Accordingly, if smoke has occurred due to a fire at the time that the above-mentioned correction and storage operation is performed, and thus the smoke detector is going to issue an alarm, then the condition where the smoke exists is stored as a memory content for a reference of comparison. Therefore, there is the possibility of the occurrence of a situation where a smoke detector fails to perform its normal operations. Thus this system is not reliable.
There has been another proposal, as disclosed in a Japanese Laid-open patent application Nos. 56,981/77 and 93,699/77, to compensate for the attenuation of the received light. This proposal successively stores the receiving light signal into a shift register and compares the received light amount at the final state of the shift register with the present amount of the received light for the fire detection. Because of the use of the shift register, however, the fire sensor does not function as a sensor from the time that power is turned on until the receiving light signal reaches the final stage of the shift register. Therefore, a long time is taken for a sensitivity test immediately after the sensor is installed.